The present invention relates to a liquid crystal display, in which light intensity is controlled by amplitude of voltage applied thereto, and in particular to a liquid crystal light valve suitable for a projection type display and a projection type display using same.
Liquid crystal displays of active matrix system using MOS (Metal Oxide Semiconductor) transistors formed on a monocrystal silicon substrate for switching elements are described in U.S. Pat. No. 3,862,360 and Technical Reports of the Electronic Communication Society, IE 80-81, 1980, in which the switching elements and a liquid crystal layer are superposed on each other and light intensity is controlled by means of the former:
All these displays are of a system, in which images obtained by controlling the switching elements are viewed directly, and they are used usually in a room. For this reason, a resistance to light intensity of several tens of thousand lx necessary for a display panel was sufficient.
When an MOS transistor is irradiated with light, photo-current is produced in PN junction portions forming a source and a drain in the MOS transistor. This current changes voltage applied to the liquid crystal, which remarkably worsens image quality.
In order to reduce this photo-current, according to the Technical Reports of the Electronic Communication Society described previously, various countermeasures were taken that the source region in the MOS transistor was located as far as possible from a region where light was injected, that the surface of the silicon substrate on which MOS transistors were formed was covered by two wiring layers, that a stopper diffusion layer was disposed to recombine generated carriers, etc.
In addition, since display size in the displays described above was as small as about 5 cm because of restriction imposed by silicon wafers, etc., the number of pixels in such a display was about 40,000 due to this display size and resolving power capable of producing recognizable images.
As described above, liquid crystal displays using MOS transistors formed on a monocrystal silicon substrate were restricted to be of direct view type.
On the other hand, in a projection type display, a panel constructed by superposing switching elements and a liquid crystal layer on each other is called a liquid crystal light valve and images controlled by this light valve are projected on a screen in an enlarged scale. For this reason intensity of light projected to the light valve should be increased, corresponding to the enlargement on the screen, and brightness thereof attains several million lx. Furthermore, since the pixels controlled by the light valve are enlarged and the images thus obtained are roughened, more than 30,000 pixels are required for the light valve.
As described above, for a projection type display, such as a liquid crystal light valve using transistors formed on a semiconductor substrate such as silicon substrate, it is required to increase resistance to light intensity of the liquid crystal light valve and to write image signals with a high speed in pixels due to increase in the number of pixels.
The present invention has been made in view of such a situation and objects thereof are to provide a liquid crystal light valve using a semiconductor substrate such as a silicon substrate, etc., which is not influenced by strong irradiation by light and excellent in the resistance to light, to provide a liquid crystal light valve, in which image signals can be written with a high speed, and further to provide a projection type display, which can display very fine and bright images of high quality by using such a liquid crystal light valve.
In order to achieve the above objects, a liquid crystal light valve according to the present invention is constructed as described below.
It comprises a semiconductor substrate having a region for a plurality of switching elements formed in a matrix form on a surface thereof; a first metal layer formed on the surface of the semiconductor substrate through an insulating layer and divided into a plurality of parts by first slits; a second metal layer formed on the first metal layer through another insulating layer and divided into a plurality of parts by second slits; a third metal layer formed on the second metal layer through still another insulating layer and divided into a plurality of parts by third slits; an opposite substrate having an opposite electrode on a surface thereof, disposed so as to be opposite to the third metal layer through an interval on the opposite electrode side; and liquid crystal filling the interval between the opposite electrode and the third metal layer, wherein the first slits, the second slits and the third slits are located so as to be displaced from each other in a direction parallel to the surface of the semiconductor substrate so that light-projected from the opposite substrate side is prevented from reaching the semiconductor substrate.
Further it comprises a semiconductor substrate having a region for a plurality of switching elements formed in a matrix form on a surface thereof; a first metal layer formed on the surface of the semiconductor substrate through an insulating layer and divided into a plurality of parts by first slits; a second metal layer formed on the first metal layer through another insulating layer and divided into a plurality of parts by second slits; an opposite substrate having an opposite electrode on a surface thereof, disposed so as to be opposite to the second metal layer through an interval on the opposite electrode side; and liquid crystal filling the interval between the opposite electrode and the second metal layer, wherein semiconductor regions connected with a reference potential are disposed at places where light injected from the opposite substrate side through the first slits and the second slits reaches the semiconductor substrate.
Still further a capacitive element region is disposed, corresponding to each of the switching element regions, on the surface of the semiconductor substrate and substrate feeding lines feeding substrate potential regions and capacitive element regions in the switching element regions described above with a substrate potential are constructed by either one of the metal layers described previously.
Still further image signal lines feeding image signal input terminal portions in the switching element regions with image signals are constructed by either one of the metal layers described previously and the substrate feeding lines and the image signal lines are disposed parallel to each other.
Since the metal layers reflect injected light, they weaken light projected to the surface of the semiconductor substrate and thus it is possible to reduce significantly photo-current flowing through the switching element regions.
Photo-current generated by irradiating semiconductor regions connected with the reference potential with light is consumed by flowing to wiring portions on the reference potential side and thus has no influence on the switching element regions. Impedance of the substrate feeding lines and the image signal lines can be reduced and speed, with which image signals are written in the different pixels, can be increased, owing to the fact that these lines are constituted by the metal layers and disposed parallel to each other.
Since the switching elements in the liquid crystal light valve are not influenced by irradiation light and it is possible to increase the number of pixels by increasing the speed with which image signals are written, it is possible to provide a projection type display capable of displaying very fine and bright images of high quality.